It is known that cholesterol is an important steroid found in the lipids (fats) in the bloodstream and in all body's cells in mammals. Cholesterol is used to form cell membranes, some hormones and other needed tissues. A mammal will get cholesterol in two ways; the body produces some of it, and the rest comes from products that the mammal consumes, such as meats, poultry, fish, eggs, butter, cheese and whole milk. Food from plants like fruits, vegetables and cereals do not include cholesterol.
Cholesterol and other fats can't dissolve in the blood. They have to be transported to and from the cells by special carriers called lipoproteins, named on basis of their density. Low-density lipoprotein, or LDL, transport cholesterol from the liver to peripheral tissues and LDL transported cholesterol is known as the “bad” cholesterol, because too much LDL cholesterol can clog the arteries to the heart and increase the risk of heart attack. High-density lipoprotein, or HDL, transport cholesterol back to the liver where surplus cholesterol is disposed of by the liver as bile acids. HDL transported cholesterol is known as the “good” cholesterol and high levels of HDL may reduce cholesterol deposits in arteries. For an organism to remain healthy, there has to be an intricate balance between the biosynthesis of cholesterol and its utilization, so that arterial deposition is kept at a minimum.
In e.g. marine oils, cholesterol is stored as “free” respectively as “bound” cholesterol. In the bound form, cholesterol is esterified on the OH-group by a fatty acid.

The commercially important polyunsaturated fatty acids in marine oils, such as fish oil, are preferably EPA (eicosapentaenoic acid, C20:5), DHA (docosahexaenoic acid, C22:6). The full nomenclature of these acids according to the IUPAC system is: EPA cis-5,8,11,14,17-eicosapentaenoic acid, DHA cis-4,7,10,13,16,19-docosahexaenoic acid. For many purposes it is necessary that the marine oils should be refined in order to increase the content of EPA and/or DHA to suitable levels, or to reduce the concentrations of, or even eliminate, certain other substances which occur naturally in the raw oil, e.g. cholesterol.
The fatty acids EPA and DHA are also proving increasingly valuable in the pharmaceutical and food supplement industries in particular. It is also very important for fish oils and other temperature sensitive oils (i.e. oils that contains long chain polyunsaturated fatty acids) to keep the load of the temperature as low as possible. Concerning the amount of cholesterol in the oils, it is specially a problem in fish oils and milk fat.
Further, as the link between high serum cholesterol levels and heart disease has become increasingly apparent, cholesterol-free and cholesterol-reduced food products have become more attractive to consumers, and food products that have no or reduced cholesterol are gaining popularity as well as an increasing share of the market. Consequently, removal or reduction of cholesterol in high cholesterol foods has the potential to substantially increase marketability and value.
The removal or reduction of cholesterol in marine oils is not a trivial matter. Several different techniques to accomplish this task have been developed, each with varying levels of success. The content of cholesterol in marine oils will become a much more important parameter for the process industry in the future.
Some methods of treating a fish oil is known from the prior art. Such methods include conventional vacuum steam distillation of fish oils at high temperatures which creates undesirable side reactions, decreases the content of EPA and DHA in the oil and the resulting product has a poor flavour stability and poor resistance to oxidation.